--input would still be your FASTQ file (reads) in this case: providing MetaPhlAn output to HUMAnN just allows you to skip the first step in which HUMAnN would otherwise run MetaPhlAn for you.
This covers the default workflow and a few common follow-up steps. Special options (like bypassing MetaPhlAn) are described in the manual but omitted from the Tutorial in the interest of brevity.